Wind energy has been harnessed for many generations, notably for powering sailing ships and windmills. Before the industrial revolution, windmills and windpumps were a familiar sight, typically converting wind energy directly to mechanical energy, and they never completely disappeared from the landscape. Wind turbines were first used to provide electric power in the late 19th century; in the latter part of the 20th century, wind farms began to appear for generating electricity in quantities sufficient, for example, to be fed into an electrical grid. While fossil fuels still appear to be plentiful, they are vulnerable to fluctuations in supply (and therefore price), often for geopolitical reasons; furthermore, accounting for their true cost would have to consider such factors as environmental degradation and defense expenditures. Wind-derived power is one attractive alternative to fossil fuels, since, in many climates, it can be produced locally.
A limitation of wind-derived power is its dependence on a variable resource. Other factors being equal, the power available from a wind turbine is proportional to the cube of the wind speed. Obviously, the lower the prevailing wind speed at a given location, the less power is available and the more costly is the generated power per unit. Furthermore, there is a threshold or startup wind speed below which most turbines are not reliably actuated because of insufficient torque. It is currently accepted that for wind-generated power to be considered at a given location, wind speeds of at least 7 miles per hour (mph) or 3 meters per second (m.s−1) are required.
Wind speeds are conveniently assigned into classes (Table 1). Each class represents a mean wind power density range (in units of W/m2) or equivalent mean wind speed at specified altitudes.
TABLE 1Wind Power ClassificationsAltitude 10 m (33 ft)Altitude 50 m (164 ft)Wind PowerSpeedWind PowerSpeedWind PowerDensitym · s−1Densitym · s−1ClassW/m2(mph)W/m2(mph)1<100<4.4 (9.8)  <200<5.6 (12.5)  2100-150 4.4 (9.8)-200-300 5.6 (12.5)-5.1(11.5)6.4(14.3)3150-200 5.1(11.5)-300-4006.4(14.3)5.6(12.5)7.0(15.7)4200-2505.6(12.5)400-5007.0(15.7)6.0(13.4)7.5(16.8)5250-3006.0(13.4)500-6007.5(16.8)6.4(14.3)8.0(17.9)6300-4006.4(14.3)600-8008.0(17.9)7.0(15.7)8.8(19.7)7>400>7.0(15.7)  >800>8.8(19.7)  
Areas designated class 3 or greater are suitable for most wind turbine applications, whereas class 2 areas are marginal. Class 1 areas are generally not suitable, although a few locations (e.g., exposed hilltops) with adequate wind resource for wind turbine applications may exist in some class 1 areas.
Much of the world, including significant areas of the United States, are in wind power classes 1 and 2, which currently are not favored for power generation. There is therefore a need for an airfoil assembly that can more efficiently harness wind energy in zones 1 and 2. In particular an airfoil assembly is needed which has a reliable startup in wind power classifications 1 and 2. There is further a need that such an airfoil assembly can be made without excessive cost and with commonly available materials.